The basal midbrain as a region modulating the level of alerting in the toad, Bufo paracnemis.

Reversible thiopental blockade of sites located in the basal midbrain of conscious toads bearing chronically implanted cannulae induced a response in which the animal withdrew to the bottom of the experimental box with flexed head and forelegs. Partial or total eyelid closure, myosis, and reduced ventilatory movements also occurred. No significant changes in arterial pressure levels or heart rate occurred and the reflex wiping response to the application of a painful stimulus (acid) to the skin was also unaffected. However, the somatic and visceral responses to nonpainful stimuli were reduced. There was a decrease, but not abolition, of forebrain waves in the 11 Hz range grouped in spindles, perhaps due to the reduced ventilatory drive. On the basis of these findings and of previous data from our laboratory and others on other nonmammalian groups, we postulate the existence of neurons involved in the modulation of behavioral and visceral alerting in the basal midbrain.